LRP1 B is a novel member of the low-density lipoprotein receptor (LDLR) family. It was discovered as a putative tumor suppressor and is frequently deleted in lung cancer cells. It shares high sequence and structural homology with another giant member of the LDLR family, LRP. Receptors in the LDLR family mediate endocytosis of a variety of extracellular and membrane ligands and participate in signaling pathways that function in cell migration and differentiation. In addition, several receptors in the family regulate endocytosis and regeneration of urokinase plasminogen activator receptor (uPAR), which plays essential roles in the cell migration and invasion of cancer cells. LRP1 B and LRP have overlapping ligand binding specificities; however, our recent studies demonstrate that LRP1 B exhibits a significantly slower rate of endocytosis when compared to LRP. As a result, expression of LRP1 B prevents the regeneration of uPAR to the cell surface and inhibits cell migration. From these studies, we hypothesize that the tumor suppressive function of LRP1B is related to its ability to retain uPAR in a non-functional state on the cell surface, and that alterations in the expression and endocytic efficiency of LRP1B may impact the level of functional cell surface uPAR, tumor cell migration, invasion, growth, and metastasis. For the current proposal, we have designed experiments to dissect the mechanisms by which LRP1B suppresses uPAR function and to examine how changes in LRP1B expression and endocytosis influence cancer cell migration and tumor growth. We propose the following specific aims: 1) define interactions between LRP1B and the components of the uPAR system, and explore the roles of LRP1B in uPAR/integrin-mediated signaling; 2) dissect the mechanism underlying the slow endocytosis rate of LRP1 B; 3) examine the roles of LRP1B in uPAR-mediated human cancer cell proliferation, migration, and invasion in vitro, and tumorigenesis and metastasis in vivo; and 4) determine whether the LRPIB gene, transcript, and protein levels are altered in human tumor tissues. Results from these proposed studies should allow us to better understand if and how modulation of LRP1B expression in cancer cells influences uPAR-mediated tumor cell behavior. Together, these studies should allow us to vigorously test whether LRP1B is a tumor suppressor, and may provide a framework for the design of therapeutic strategies targeting LRP1B expression and function in human cancers.